1. Field of the Invention
2. Description of the Related Art
In wireless communication systems, particularly in mobile radiotelephone systems that do not allow time domain duplexing (TDD), two different frequency bands are usually provided that serve as a transmission (Tx) band and a reception (Rx) band from the point of view of the communication participant. A common antenna is used for the transmission and reception of signals on the communication transmission device, particularly in the mobile radiotelephone device (cell phone). A duplexer is generally needed for the separation of transmission and reception signals, this being connected between antenna and transmission and reception path. Such a duplexer is primarily composed of two interconnected filters, namely an Rx filter between the antenna and the Rx path (a low noise amplifier-LNA) for received signals, and a Tx filter between the Tx path (power amplifier-PA) and the antenna for signals to be sent.
Since the communication terminal device must be able to simultaneously transmit and receive, each of the two filters must be able to adequately suppress a signal lying in the other frequency band. Typical values that are required in such wireless communication systems, for example, for the suppression of the Tx band by the Rx filter, lie in the range around 50 dB and above. At the same time, the respective signals must experience only minimal losses when passing through the filter in the respective frequency band. A typical value for a maximally tolerable attenuation of the Tx band in the Tx filter is 2 dB or better.
Known duplexers that meet these high demands made by the requirements for band separation (stop band suppression) and insertion attenuation are constructed, for example, of microwave ceramic filters. Given an adequate band spacing of the Tx and Rx band, surface-active wave filters (SAW filters) can also be employed. When, however, the Tx and Rx bands lie extremely close to one another, it is very difficult or even impossible to simultaneously meet all demands with SAW filters by themselves. One example of such a system is the American CDMA/TDMA-1900 (according to IS-95 or IS-136) in which the transmission and reception band are respectively 60 MHz wide and which provides a duplex spacing (constant spacing between transmission and reception signals) of 80 MHz. Given this bandwidth, a spacing of only 20 MHz remains between the two bands, this corresponding to approximately 10000 PPM for this frequency band. Within only 20 MHz, the respective filter must switch from the pass band with, for example, 2 dB attenuation into the stop band where, for example, the 50 dB attenuation is required. This requires a transmission behavior that exhibits steep signal edges in the pass band. Since a temperature-dependent frequency drift of the filter as well as manufacturing tolerances must also be additionally considered, it was previously not possible to construct such a duplexer only on the basis of the SAW filter technique. This required SAW filters that comprised a pass band with extremely steep signal edges.